WIP trying GCC 7 on macOS

Author: mgeier

.travis.yml

@@ -82,7 +82,7 @@ matrix:
     - os: osx
       osx_image: xcode10.1
       env:
-        - MATRIX_EVAL="brew install gcc@8 && CC=gcc-8 && CXX=g++-8"
+        - MATRIX_EVAL="brew install gcc@7 && CC=gcc-7 && CXX=g++-7"
     # osx with xcode10.1/clang
     - os: osx
       osx_image: xcode10.1